Crystal structure, vibrational and thermal behavior of Ba(NH4)[Co(CN)6]·4H2O: a new precursor for the synthesis of hexagonal BaCoO3

DIEGO M. GIL; ALEJANDRO DI SANTO; FERNANDO POMIRO; G. A. ECHEVERRÍA; O. E. PIRO; RAÚL E. CARBONIO; AIDA BEN ALTABEF

POLYHEDRON

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2014 vol. 73 p. 51 - 58

0277-5387

The crystal structure of Ba(NH4)[Co(CN)6].4H2O complex was determined by X-raydiffraction methods. It crystallizes in the hexagonal P63/m space group with a = b = 7.6882(2)Å, c = 14.4764(4) Å, and Z = 2 molecules per unit cell. The structure was solved from 475reflections with I>2(I) and refined to an agreement R1-factor of 0.0214. The cobaltocyanideanion has an octahedral shape with its Co(III) ion sited at a crystallographic special positionof point symmetry 3 (S6) [d(Co-C) = 1.886(3) Å and d(C-N) = 1.143(4) Å]. The barium ion isin a 9-fold environment at a crystal special position 6 (C3h), coordinated to cyanide N-atomsof neighboring complexes [d(Ba-N) = 2.862(3) Å] and to one type of water molecule layingonto an m mirror plane [d(Ba-Ow) = 2.924(4) Å]. The ammonium ion and the other type ofwater molecule share four symmetry-related C3 lattice sites with equal occupancy. Under theP63 subspace group these four sites split into two independent pairs of C3 sites, one occupiedby two symmetry-related ammonium ions, the other by two (disordered) water molecules. Thecomplex was characterized by UV-Visible, IR and Raman spectroscopy and the thermaldecomposition was studied to investigate the formation of BaCoO3-δ. This compound, in itshexagonal form, could be synthesized at temperatures as low as 675o C. At 750o C a neworthorhombic poly-type is obtained in a mixture with the hexagonal form which is associatedwith the appearance of oxygen vacancies. These temperatures of synthesis and reaction timesare lower than those reported by ceramic and sol-gel synthesis. These soft treatments gave riseto homogeneous particles of small grain size.